A typical paper feed mechanism has an endless, flexible and relatively inextensible sprocket driven belt. The belt has a row of uniformly spaced feed pins extending perpendicularly relative to the outer surface of the belt. The belt has drive teeth around its inside surface and is entrained around to form trace regions between a pair of spaced drive members, such as sprockets, with grooves that mesh with the drive teeth. The paper has one or more rows of uniformly spaced perforations and is driven by the feed belt whose pins are aligned with the row of perforations and have the same spacing so as to enter the perforations near one end of the trace and withdraw from perforations at the other end of the trace. The belt and the length of the trace regions are designed so that several pins simultaneously engage the perforations and drive the paper within the perforations. The sprocket and drive belt assembly are supported between a pair of frame members into which the sprockets are journaled. In some tractors, the drive mechanism comprises a single sprocket and a shoe around which the belt is wound, the shoe being essentially non-rotatable. One problem with the shoe structure is that it is stationary and thereby imposes increased frictional loads and wear on the belt as it slides over the shoe.
The belt is supported in the trace regions between the sprockets by a support structure attached to or made part of one or both of the side frames. The support structure has a horizontal support surface which engages and supports the underside of the flexible belt over at least that portion of the trace distance where several of the feed pins will be within the perforations and in full drive engagement with the paper. The support surface may be a continuous curve or may have a flat portion between upward and downward linear ramps. The gear teeth ride on the support surface. It is common to provide the horizontal support surface with a longitudinal guide slot or channel in which either the gear teeth on the underside of the belt or both gear teeth and belt can enter and be guided by the side walls of the channel. The purpose of the channel is to limit the lateral movement of the belt in the trace region in order that belt and hence the feed pins are maintained in alignment with the perforations in the paper during entry, withdrawal and full engagement with the paper. A problem with previous tractor designs made of plastic was that friction between the guide structure and the belt caused excess loading and wear of the belt thereby causing misalignment of the pins with the perforations of the paper.
High speed printers use as many as four tractors supported on parallel guide and drive shafts of a carriage mechanism. The tractors are movable transversely on the guide shafts to obtain proper alignment of the feed pins with the perforations along the edges of the paper. The adjustment mechanism may comprise lead screws or cables connected to the tractors in a manner whereby all or some of the tractors can be moved laterally along the shafts. In the past, tractor designs were such that many different parts were needed for the different tractors.
Originally tractors were made largely from metal parts to assure long wear and precision required for the feed pins to be aligned with and to enter and withdraw from the perforations without damage to the paper. Such tractors tended to be costly and required a large number of different parts especially to provide tractors for feeding along both edges of the forms. Later tractors were made with parts made almost entirely with plastic materials which could be fabricated by casting or molding processes. The belt was also made entirely of plastic materials molded entirely as a single endless belt or alternatively from a thin strip of plastic or metal with plastic drive elements molded thereto. Examples of such tractors using various plastic materials are shown in U.S. Pat. Nos. 3,825,162; 3,930,601; 4,226,353; 4,457,463 and 4,614,287.
Some of the problems associated with plastic tractors have been lack of adequate structural strength, dimensional instability and rapid wear which affect proper alignment of the belt with the paper and the pins with the perforations. Another problem has been the build up of electrical charge which makes the paper cling to the forms path and resist refolding.
It has been common practice to apply a tensioning force to the belt to maintain a taut pin belt during assembly and operation. Such use has been primarily in tractors where the belt is wrapped around one sprocket and a guide shoe. The guide shoe is mounted between two frame plates so as to be movable therebetween relative to the drive sprocket which is journaled to the frame plates. A compression spring applies pressure to the movable shoe within the space between the sides of the frame members. It is common to lock the shoe in place after the belt has been stretched taut by the spring. As a result of wear through use, the belt eventually develops some slack thereby causing misalignment of the pins and loss of paper control. The shoe is then unlocked so as to allow movement of the shoe relative to the sprocket to thereby restore the necessary amount of tautness to the belt. A problem with prior belt tensioning structures is the tendency of the tensioner to become jammed against the frame members so that when released it will not move solely under the force of the spring to restore the belt to its initial tautness. One reason for this is the lack of adequate space for the tensioner within the tractor. Another is paper dust lodging within the spaces occupied by the tensioner. Yet another is that the materials used tend to change shape or size thereby causing adherence to the frame elements. Some kind of manual operation is then required to unfreeze the shoe from the frame members. Examples of tractor belt tensioning devices are shown in U.S. Pat. Nos. 3,930,601; 4,226,353; 4,453,660; 4,457,463; 4,462,531; 4,638,935 and 4,723,697 and in articles published in the IBM Technical Disclosure Bulletin, Vol. 16 No. 1, Jun. 1973, p. 309; and Vol. 22 No. 7, Dec. 1979, pp. 2636-2637.